1. Field of the Invention
The present invention relates to an electrode material and an electrode for a III-V group compound semiconductor.
2. Description of the Related Art
III-V group compound semiconductors expressed as a general formula of InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6zxe2x89xa61 are applied to light emitting device ultra-violet or blue light emitting diodes and ultra-violet or blue laser diodes. Properties of these compound semiconductors have, however, hardly been elucidated, and no electrode materials having low contact resistance against the p-type III-V group compound semiconductors have been obtained yet.
Development of electrode materials having low contact resistance against the p-type III-V group compound semiconductors is highly demanded in order to realize light emitting devices of favorable electric properties.
One object of the invention is thus to provide an electrode material having the low contact resistance against a III-V group compound semiconductor, which is expressed as a general formula of InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61, and doped with p-type impurities.
Another object of the invention is to realize a light emitting device having a high luminance and driven at low voltages.
As a result of extensive studies, the inventors have found that an alloy of Au and a specific metal has a sufficiently low contact resistance against the III-V group compound semiconductors.
The invention is directed to an electrode material applied to a III-V group compound semiconductor, which is expressed as a general formula of InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61, and doped with p-type impurities, the electrode material comprising an alloy of Au and at least one metal selected from a group consisting of Mg and Zn.
Also the invention is directed to an electrode applied to a III-V group compound semiconductor, which is expressed as a general formula of InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61, and doped with p-type impurities, the electrode formed with the afformentioned material.
The electrode material of the invention is applicable to III-V group compound semiconductors which are expressed as a general formula of InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61, and doped with p-type impurities.
Such III-V group compound semiconductors are prepared, for example, by molecular beam epitaxy (hereinafter referred to as MBE), by organometallic vapor phase epitaxy (hereinafter referred to as MOVPE), or hydride vapor phase epitaxy (hereinafter referred to as HVPE). Gas source molecular beam epitaxy (hereinafter referred to as GSMBE) is generally applied among the MBE processes, where nitrogen gas, ammonia gas, or another gaseous nitrogen compound is supplied as nitrogen source. The nitrogen source may be chemically inactive, which prevents nitrogen atoms from being easily taken into the crystal. Under such conditions, the nitrogen source may be excited and activated by a method, for example microwave excitation, thereby enhancing the intake efficiency of nitrogen atoms.
In the case of MOVPE, using so called 2 step-growth method, where a buffer layer such as thin film of AlN or GaN is grown on the substrate and the compound semiconductor is grown on the buffer layer, is effective to obtain the compound semiconductor of good crystallinity (Applied Physics Letters, vol. 48 (1986), p 353, Japanese Journal of Applied Physics, vol. 30 (1991), p L1705).
Examples of the p-type impurities applicable to the invention include Mg, Zn, Cd, Be, Ca, and Hg. Preferable impurity atoms are Mg and Zn, and more specifically Mg.
The p-type impurities are given from the materials described below.
As long as a simple body of dopant is controlled to a vapor pressure which does not interfere with other molecular beams in a device of manufacturing the III-V group compound semiconductor by the process of GSMBE, the simple body of dopant is directly applied as the p-type impurity.
Volatile materials given below may be applied in the process of MOVPE.
Examples of material for giving zinc atoms (Zn) include alkylzincs expressed as a general formula of R1R2Zn, where R1 and R2 are, independently from each other, alkyl groups having 1 to 4 carbon atoms, such as dimethylzinc (CH3)2Zn and diethylzinc (C2H5)2Zn.
Examples of material for giving magnesium atoms (Mg) include bis(cyclopentadienyl)magnesium (C5H5)2Mg (hereinafter referred to as Cp2Mg), bis(methylcyclopentadienyl)magnesium (CH3C5H4)2Mg, and bis(isopropylcyclopentadienyl)magnesium (i-C3H7C5H4)2Mg.
Examples of material for giving cadmium atoms (Cd) include alkylcadmiums expressed as a general formula of R1R2Cd, where R1 and R2 are, independently from each other, alkyl groups having 1 to 4 carbon atoms, such as dimethylcadmium (CH3)2Cd.
Examples of material for giving beryllium atoms (Be) include diethylberyllium (C2H5)2Be and bis(methylcyclopentadienyl)beryllium (CH3C5H4)2Be.
Examples of material for giving mercury atoms (Hg) include alkylmercuries expressed as a general formula of R1R2Hg, where R1 and R2 are, independently from each other, alkyl groups having 1 to 4 carbon atoms, such as dimethylmercury (CH3)2Hg and diethylmercury (C2H5)2Hg.
The electrode material of the invention applied to III-V group compound semiconductors is an alloy of Au and at least one metal selected from the group consisting of Mg and Zn. Concrete examples are Auxe2x80x94Mg, Auxe2x80x94Zn and Auxe2x80x94Znxe2x80x94Mg alloys.
Preferable concentration range of Mg in the electrode material is 0.1 to 2.5% by weight. Preferable concentration range of Zn in the electrode material is 1 to 30% by weight.
The electrode material of the invention has the low contact resistance against the III-V group compound semiconductor, thereby realizing a light emitting device having a high luminance and driven at low voltages.
The electrode material applicable for the III-V group compound semiconductor is prepared in the following manner.
At a first step, a III-V group compound semiconductor, which is expressed as a general formula of InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61, and doped with p-type impurities is grown according to a known method. The electrode material of the invention is then vacuum-deposited onto the compound semiconductor. It is preferable to vacuum-deposit the electrode material of the invention after the III-V group compound semiconductor is irradiated with electron beams or heated to 500xc2x0 C. or higher temperatures for annealing.
Any known method is applicable to vacuum deposition. For example, an Auxe2x80x94Zn alloy is vacuum-deposited on the compound semiconductor with a tungsten boat by the resistance heating process.
In another application, a semiconductor laminate formed by vacuum deposited Au and a specific metal on the compound semiconductor is annealed to form an alloy on the semiconductor.
In some cases, annealing improves the electric contact.
An atmosphere applied for annealing may be sufficiently purified nitrogen gas or inert gas such as argon. The temperature of annealing is preferably in a range of 200xc2x0 C. to 1,000xc2x0 C. or more specifically in a range of 300xc2x0 C. to 900xc2x0 C. The lower annealing temperature results in insufficient effects whereas the higher annealing temperature causes denaturation of constituents of the semiconductor device so as to deteriorate the properties of the semiconductor device.
The time period for annealing is determined according to the annealing temperature, but is preferably in a range of 1 second to 2 hours or more specifically in a range of 2 seconds to 30 minutes. The shorter annealing time results in insufficient effects whereas the longer annealing time causes denaturation of constituents of the semiconductor device so as to deteriorate the properties of the semiconductor device or worsen the productivity.
Another metal may further be laid over the electrode which is in contact with the III-V group compound semiconductor. This enhances the mechanical strength and other mechanical properties without damaging the electric properties.
A concrete example of an electrode, which has the above mentioned laminate structure, is schematically shown in FIG. 1, where the second metal electrode 3 is deposited onto both surfaces of the electrode of the invention 2 and of the compound semiconductor 1. The examples of the metals which may be used as the second electrode are, Al, Ti and Cr. Among these metals, Al may be preferably used because of its good adhesion properties to the compound semiconductor.
In general, an electric contact is made to the second electrode.